01-03-2013, 12:17 PM
A Nanotechnological Approach to Contaminated Water Treatment
A Nanotechnological.docx (Size: 38 KB / Downloads: 35)
ABSTRACT
Scarcity of water, in terms of both quantity and quality, poses a significant threat to the current and future well-being of people worldwide, but especially to people in developing countries. Sustainable water management is a critical aspect of addressing poverty, equity, and related issues. Science and technology has a role to play in contributing to the development of new methods, tools, and techniques to solve specific water quality and quantity problems. Projects that meet economic, social, and environmental criteria can contribute to sustainable management of water resources and improve access to clean water for poor people in developing countries. This paper provides an overview of water treatment devices that incorporate nanotechnology; some of these are already on the market while others are still in development. The paper then explores potential environmental and health risks, risk governance issues, and socio-economic issues regarding the potential use of nanotechnology to improve access to clean water and basic sanitation.
In exploring whether and how nanotechnology could be applied responsibly to offer new and better solutions, this paper describes a range of available products and promising research that apply nanotechnology to provide clean water. In particular, this paper describes nano-filtration membrane technologies used to clean water. It then describes socioeconomic issues and potential environmental and human health risks of using nanotechnology to clean water. Here we demonstrate the differences in access to technology, field conditions, and the types of technologies that may be appropriate in different circumstances
Water Pollution and Nanofiltration
Water that does not meet drinking water standards should be treated to ensure that the health of the consumer or community is not compromised through exposure to toxic pollutants. Polluted water is often treated by conventional or pressure-driven membrane processes to make it comply with drinking water standards. The conventional water treatment process consists of several stages. These include pre-treatment, coagulation, flocculation, sedimentation, disinfection, aeration, and filtration. The pre-treatment stage removes suspended solids. Coagulation and flocculation are carried out to precipitate dissolved impurities through sedimentation. The water is then filtered to remove any suspended particles. One of the disadvantages of the conventional water treatment method is that it cannot remove dissolved salts and some soluble inorganic and organic substances.
Pressure-driven membrane technology is an ideal method for the treatment of water to any desired quality. The integral part of the technology is the membrane. The membrane is a barrier that separates two homogenous phases. It allows some solutes to pass through but rejects the permeation of others. It achieves the separation of solutes of a fluid mixture when a driving force is applied. The force could be a pressure difference (Æp), concentration gradient (Æc), temperature difference (ÆT), or electrical potential difference (ÆE).The basic principle of operation is illustrated in Figure 1. Phases 1 and 2 are generally the feed water and the product water or permeate, respectively. The basis of separation is that each membrane has unique characteristics for the selective permeation and rejection of different
desalination
Desalination is the removal of dissolved salts from raw or untreated water by either thermal or membrane processes. A thermal process uses heat to evaporate water, which is then collected by condensation. In a membrane process, pressure is applied to force the raw water through a membrane that retains the dissolved salts. Reverse osmosis (RO) membranes can retain all the salt, whereas other membrane processes, such as nanofiltration (NF), selectively retain some salts. Desalination is carried out for various reasons, including limited freshwater, increasing demand, global warming, regulation, cost effectiveness, and politics. A reverse osmosis (RO) desalination plant consists of the following sequence of stages: feed water intake system, pre-treatment facility, high-pressure feed pumps, RO membrane, and desalinated water conditioning system. A pressure of 40 – 80 bars is required for the permeation of water through the RO membrane for the desalination of seawater. Two membrane sheets are glued together and spirally wound around a perforated central tube. The product water exits through this tube. Nanotechnology is used in Israel for the desalination of saline waters. The Grand Water Research Institute of the Israel Institute of Technology is working with corporate and other partners to treat salt water and create fresh sources for drinking water and irrigation.
Suitability of the Nanomembrane Technologies
There are merits and demerits of nanomembrane technologies (e.g., nanofiltration and reverse osmosis) over conventional filtration technologies. The conventional sand filter does not retain some microbes and dissolved salts (e.g., arsenate). Nanofiltration (NF) and reverse osmosis (RO) membranes remove all multivalent ions and bacteria. The conventional carbon filter, biological sand, and biological carbon filters do not remove some bacteria and dissolved salts (e.g., calcium). Calcium is readily removed by the nanomembrane processes. However, NF membranes have a low rejection coefficient ® for monovalent ions. Thus, they are not well suited for the removal of nitrate and fluoride ions from water